As you said, it depends on the particular license of the software. The license really goes a long way in terms of what you're allowed to do or not, and many debates ensue as to which open source license really respects the spirit and ideals of the OSS movement. Even the FSF actively maintains two versions of the GNU License (GPL and LGPL which are very different). As Josh kelley points out, the Free Software Foundation has a whole bunch of stuff related to this philosophies (as Linus Torvalds put in once, Stallman is the philosopher and he's the engineer.
However, the very definition of open source software, as broad as the term can be, is very straight-forward. Can you legally see the source code and modify it? Then it's open source. Is this documented somewhere? I'm very sure it is, be it a dictionary, an encyclopedia or the documentation of an open source license. Take your pick.
Open source licenses deal with distribution details of both the original work and your contributions, they already assume that the source code is open to see and modify.